1. Field of the Invention
The present invention generally relates to a low-voltage detecting circuit for detecting the low-voltage state of power sources, and more particularly, to a low-voltage detecting circuit for determining whether the measured voltage value is insufficient by detecting the discharge time of a capacitor.
2. Description of the Prior Art
It has been an essential issue and technique in the field of electrical/electronic engineering to determine whether the power source of a circuit device or the electromotive force of a battery is in a loaded state. Especially in the recent years, the wireless electronic devices, such as cellular phones and wireless keyboards, have been widely used in commercial applications. Consequently, batteries necessarily serve as the sources to provide the needed power. When the user uses a battery as the power source for a wireless electronic device, it is important to determine whether the electromotive force of a battery is sufficient, so as to remind the user of replacing a new battery. Accordingly, there is a need for a low-voltage detecting circuit for detecting the state of a battery.
To date, the voltage detection precision is 0.1V in the prior art for the imported CMOS-based low-voltage detecting circuits. The disadvantage of such low-voltage detecting circuits is that such circuits generally dissipate a large current as 1xcx9c2xcexcA (3V). Furthermore, the price is high because the market is monopolized. Therefore, there exists a need for providing a cheaper and simplified technique for the implementation of a low-voltage detecting circuit for use in wireless devices.
The prior art technique can be exemplified by, for example, U.S. Pat. No. 5,838,173 xe2x80x9cDevice and method for detecting a low-voltage in a system,xe2x80x9d as shown in FIG. 1, wherein the conventional circuit includes an oscillator 9 connected to a low-voltage detecting signal generator 19 to output a waveform according to the variation in the transconductance of transistors. The oscillator 9 generates a rectangular waveform and the number of the pulses in the waveform is proportional to the voltage level of the power source, and then is counted by the counter register 29 in the low-voltage detecting signal generator 19, which is used to generate a low-voltage detecting signal in response to the output of the oscillator 9. As the source voltage Vcc gets higher, the number of the pulses is increased in a constant period and the frequency of the waveform is also increased. Finally, the voltage level of the power source can be detected by being compared to a reference value.
The problem according to the conventional low-voltage detecting circuit is that the current dissipation exists even when the detecting circuit is not in its detecting operation. Furthermore, the fact that the circuit design in the conventional detecting circuit utilizes a great amount of electronic elements results in a increased fabrication cost. Accordingly, the conventional low-voltage detecting circuit is complex, expensive, power dissipating, and required to be improved.
In order to overcome the above problem, the present invention provides a low-voltage detecting circuit for power sources to reduce the cost of the circuit, simplify the circuit design, and further eliminate the power dissipation for power saving. The principle of the present invention is based upon the relation of the discharge time of a capacitor and the measured voltage value to determine whether the voltage level of the power source is too low.
Accordingly, it is a primary object of the present invention to provide a low-voltage detecting circuit for power sources to detect the low-voltage state and determine whether the electromotive force is sufficient to further provide a simplified detecting circuit able to detect the low-voltage state , resulting in a reduced fabrication cost.
Moreover, it is another object of the present invention to provide a low-voltage detecting circuit for power sources to reduce the power dissipation by eliminating current dissipation when the detecting circuit is not in its detecting operation.
The present invention is characterized in being based upon the formula of the discharge time of a capacitor, which can be expressed as:
V(t)=Vbexe2x88x92t/RCxe2x80x83xe2x80x83(1)
Wherein Vb indicates the voltage value of the power sources to be measured, and V(t) is the voltage of the capacitor in the discharge process. The values of the resistor and the capacitor are fixed during the process within which the V(t) discharges to a constant voltage Va, and the amount of the discharge time can be expressed as:                     t        =                  RC          ⁢                      xe2x80x83                    ⁢          ln          ⁢                                    V              b                                      V              a                                                          (        2        )            
The formula (2) is the basic principle used in the present invention to detect the voltage value of the power source by calculating the discharge time. For example, when the voltage value decreases from 3.2V to 2.7V, the detecting circuit outputs a signal or drives a light emitting diode (LED) to remind the user to replace the power source.
In order to accomplish the foregoing objects, the present invention provides a low-voltage detecting circuit, comprising: a switching module; a charge/discharge module, connected to said switching module to execute charging operation or discharging operation by grounding under the control of said switching module; a discharge detecting module, connected to said charge/discharge module to output an informing signal when said charge/discharge module discharges to a first pre-determined voltage.
It is preferable that the present invention provides a low-voltage detecting circuit, further comprising: a control module including a timer, wherein said control module is connected to said switching module and said discharge detecting module.
The operation of said low-voltage detecting circuit includes the following steps:
(a) Said control module outputs a first control signal to said switching module to drive said charge/discharge module to execute charging operation.
(b) After a pre-determined period, said control module resets said timer to zero and activate said timer; meanwhile, said control module outputs a second control signal to said switching module to drive said charge/discharge module to execute discharging operation.
(c) When said charge/discharge module discharges to a first pre-determined voltage, said discharge detecting module outputs a first signal to said control module to stop the timing calculating of said timer and outputs a calculated value.
(d) The calculated value is compared to a pre-determined value.
(e) After (a) to (d) being repeated until the calculated value reaches the pre-determined value, said control module outputs an informing signal.
It is preferable that the present invention further provides a method for detecting the low-voltage state for power sources, comprising the steps of:
(a) generating a voltage across the capacitor equal to that of the power source;
(b) discharging the capacitor by grounding and activating a timer simultaneously;
(c) stopping the timer when the capacitor discharges to a first pre-determined voltage; and
(d) comparing the ;calculated value from the timer to a pre-determined value, and outputting an informing signal when the calculated value reaches the pre-determined value.
It is preferable that the present invention provides a low-voltage detecting circuit that can serve as a low-voltage detecting circuit for the power source used in cellular phones when said low-voltage detecting circuit is implemented together with a micro-controller of cellular phones and an LED-based circuit to calculate the discharge time of the capacitor.
It is preferable that the present invention provides a low-voltage detecting circuit that can serve as a low-voltage detecting circuit for the power source used in wireless keyboards when said low-voltage detecting circuit is implemented together with a controller of wireless keyboards and an LED-based circuit to calculate We discharge time of the capacitor.